The introduction of the Closing Auction Session (CAS) for futures and options (F&O) stocks has led to a substantial decline in average daily F&O turnover on major Indian exchanges, reaching multi-month lows in August 2026. This new mechanism, which converts the final 15 minutes of trading from continuous order matching to a batch-auction system, has fundamentally changed the end-of-day trading landscape.
Significant Drop in August Turnover
Data from BSE reveals a sharp 33.87% reduction in average daily notional turnover in August, falling to Rs 1,53,55,058 crore from Rs 2,32,19,889 crore in July. This marks the lowest average daily F&O turnover for BSE since July 2025.
Similarly, the National Stock Exchange (NSE) also experienced a decrease in its daily equity derivatives turnover. In August, it stood at Rs 1,93,34,505 crore, a 9.77% drop from Rs 2,14,29,872 crore in July. This represents NSE's lowest average daily turnover since February 2025.
Impact on Trading Strategies and Participants
According to Hitesh Tailor, a Technical & Derivative Analyst at Choice Broking, the CAS rollout has particularly affected high-frequency trading strategies. These include proprietary arbitrage, index options scalping, and delta-hedging strategies, all of which rely heavily on continuous visibility of order flow.
Furthermore, brokerages have tightened their intraday auto-square-off timelines. This adjustment has effectively shortened the active trading window available to retail participants, adding another layer of complexity to their trading activities.
Compounding Factors and Future Outlook
The impact of CAS has been exacerbated by existing market conditions, including higher transaction costs and stricter margin requirements. These factors have already been curtailing speculative leverage and the participation of short-term traders in the market.
Initial figures for September show mixed results, with BSE's average daily F&O turnover further decreasing to Rs 1,30,43,750 crore after two trading sessions, while NSE's improved to Rs 3,42,68,095 crore on average for the same period.
Despite the near-term disruption in trading volumes, analysts believe the broader objective of CAS—to enhance price discovery and mitigate end-of-day volatility—will ultimately prevail. As traders, brokers, and automated execution algorithms adapt to the post-3:15 PM auction environment, F&O activity is expected to gradually stabilize and find a new normal.